In recent years, it has proved to be the case that the clock frequencies of microprocessors are increasing steadily. Clock frequencies have now passed the 1 GHz mark and a further increase in the clock frequency to 5 to 10 GHz is to be reckoned with in the next few years. The system clock on circuit boards, for example a system circuit board (so-called motherboard) of a computer, lags behind the clock of the microprocessors by orders of magnitude. Although the internal clock rate of the individual semiconductor components (so-called chips) is considerably higher than the system clock on the circuit boards, the clock rate is limited by present wiring and connection technology using copper conductors. In particular, in conventional electronic assemblies the parasitic inductances and capacitances of the copper conductor paths retard the speed at which the chips can process the data internally. In wiring terms it is becoming ever more difficult with copper conductors to gain control of crosstalk due to the radiation of high-frequency signals from the conductors, which act like antennae; attenuation losses blur the rectangular pulse shapes and make detection more difficult at the other end of the connection line. The production of a circuit board which still operates smoothly at a clock rate of 4 to 8 GHz will only be possible—if at all—in several years, and only at very great expense.
One possibility for increasing the system timing on the circuit boards would be to increase the bus width and transmit the same quantity of data at a lower speed on the individual lines due to greater parallelism. However, that would not only result in a multiplication of the number of conductors, which can scarcely be accommodated on the circuit boards even now, but also in an increase in the number of I/O connections on the chips. The possibilities for increasing the connection density offered by the transition from edge board to two-dimensional contacting will soon reach their limit, however. The alternative, namely to reduce the number of terminal pins again by a higher data rate per pin, can only be managed by a new technology in internal connection systems.
Against this background, many companies and research institutes are pursuing an approach to a solution that has as its objective data transmission via planar optical waveguides, which are integrated into conventional circuit boards. Optical connections are insensitive to high-frequency and electrostatic fields; they overcome the problems of electric line attenuation at high frequencies, crosstalk and electromagnetic compatibility (EMC) incompatibilities. In addition, data rates of 10 Gbit/s and more are possible using optical connections.
In the context of the “Electrical Optical Circuit Board” (EOCB) project sponsored by the Bundesforschungsministerium (Federal Ministry for Research), a method has been developed at the Fraunhofer Institut für Zuverlässigkeit und Mikrointegration (Fraunhofer Institute for Reliability and Microintegration) (IZM) in Berlin for networking transmitters and receivers by means of optical waveguides on an opto-electrical circuit board, which method permits one of the metallizing levels of a conventional multi-layer electrical circuit board to be replaced by an optical waveguide structure. For this purpose, the waveguides, which have a typical cross-section of 60 micrometers×60 micrometers, are first hot-stamped in an organic film material using a stamping die. The troughs are then filled with a waveguide material that has a slightly higher refractive index than the film material, and then covered with a second film. This waveguide structure can then be incorporated into a circuit board. The distance between the optical waveguides is approx. 250 micrometers.
The planar waveguides produced according to the method described above have the disadvantage, however, that the optical attenuation of the waveguides for a wavelength of 850 nm lies in the range from 0.2 to 0.5 dB/cm. For wavelengths in the range from 1300 to 1550 nm, which are normal in communications engineering, the attenuation is even higher. The reason for this attenuation is to be found primarily in the absorption of the waveguide material, scattering losses due to impurities in the waveguide material and the roughness of the boundary transition between the core and sheath of the waveguide. To produce an opto-electrical circuit board for so-called backplane applications, distances of over 50 cm have to be bridged. In the case of waveguide attenuation of 0.2 dB/cm, the total attenuation of the waveguide is then over 10 dB plus the losses due to coupling to the optical components. For applications with parallel-optical-link (POL) modules, the available capacity would thus already be exceeded.
A further disadvantage of planar waveguides is manifested in additional attenuation in the area of crossings. Crossings are necessary in the case of pure distribution tasks and in more complex applications. The interruption of the waveguide sheath at a crossing leads to divergent beam guiding, due to which a portion of the light passes from a first waveguide into a second waveguide crossing the first waveguide and there passes into the sheath, i.e. it is absorbed. Light is also scattered at the edges of the waveguide in the area of a crossing, due to which a portion of the light passes from the first waveguide into the crossing second waveguide and is guided onwards there. This results in crosstalk from the first waveguide to the second waveguide. If several waveguides cross a first waveguide, the crosstalk adds up and the signal/noise ratio at the optical receiver at the end of the first waveguide is reduced.
It is known furthermore from the prior art to produce an optical waveguide structure from glass and/or plastic fibers by means of so-called optical multiwire technology. In the multiwire method, glass or plastic fibers are laid on a substrate. The glass and/or plastic fibers run over one another in the area of crossings of the optical waveguide structure, so that light cannot spread from one optical waveguide into the crossing optical waveguide. In addition, the optical attenuation of glass and plastic fibers is very low.
However, the optical multiwire method has the disadvantage that no optical waveguide structures with branchings and connections can be produced with it.
From an article “Mikrostrukturen für die optische Kommunikation” [Microstructures for optical communication] by Professor Dr. Andreas Neyer, which appeared in the journal Funkschau 24 (1999), P. 76 to 78, a so-called SIGA (silicon microstructure, plating, casting) process is known for producing an optical waveguide structure of the type stated at the beginning. The SIGA process was developed in particular for optical waveguide structures which comprise both planar waveguides and glass and/or plastic fibers, and where highly accurate alignment of the core of the optical fiber and the core of the waveguide matters. However, the question of where planar waveguides and where glass and/or plastic fibers should be used inside a waveguide structure of this kind is not addressed in this article, nor can any allusions to this question be gathered from this article.